A piezoelectric element such as a crystal oscillator used in various electronic equipments such as a mobile telephone is used in a condition encapsulated in a package so as to prevent oxidation and deterioration by moisture and oxygen in the air. As a method of hermetic sealing of this piezoelectric element package, a brazing method is commonly used. Hermetic sealing by the brazing method is performed by fixing a piezoelectric element inside the base member which serves as a package container, stacking a brazing material molded in accordance with the shape of the bonded part and a cap member which serves as a lid, heating and melting the brazing material and thereby bonding the base member and the cover member.
Fluxless brazing materials are used as brazing materials for package sealing so as to prevent contamination to the piezoelectric element, and among such materials, Au—Sn based brazing materials which are excellent in reliability and corrosion resistance are commonly used. And the working temperature at the time of sealing is often set to 300° C. or more in consideration of a melting point of the brazing material (The melting point of an Au—Sn brazing material is around about 280° C.).    Patent Document 1: Japanese Patent Application Laid-Open No. 2005-347851